Power amplifiers that have a low power consumption while at the same time being highly efficient are required within the transmission path in mobile communication devices, in particular at the output of the transmission path. The power amplifiers should be able to amplify both signals having a substantially constant envelope and signals having a variable envelope. Signals having a constant amplitude or envelope respectively are used, inter alia, in the Bluetooth and GSM mobile communication standards. Signals in which the information to be transmitted is encoded in both the phase and in the amplitude are known for example from the mobile communication standards EDGE, UMTS/WCDMA and all WLAN standards.
In order to meet the various needs, it is possible to provide different power amplifiers optimized for the different mobile communication standards. However, the increasing integration density and the need to occupy less space while at the same time reducing power consumption would appear to necessitate the use of just one power amplifier for a multiplicity of possible mobile communication standards.
In a conventional amplifier that can be used for a plurality of different modulated signals, a variable load having a number of capacitors is connected downstream of the amplifier. Said capacitors are connected firstly to the output of the amplifier and also to a reference potential terminal. If the output power is high, the individual capacitive elements are activated, thus changing the load across the amplifier.
In the embodiment shown in the publication, however, part of this power is lost via the switching elements for connecting or disconnecting the individual capacitances, particularly if the output power is high. The amplifier consequently has to provide a higher output power. Moreover, the switches designed for a high output power require a relatively large chip area, resulting in an increase in the overall space required.